Polyol esters (POEs) are well known in the art as lubricants for displacement type refrigeration systems. Commonly used commercial POEs are derived from the reaction of a polyol (an alcohol containing 2 or more OH groups) with a monofunctional carboxylic acid. Such “simple” or “traditional” polyol esters are especially suited for use in systems utilizing hydrofluorocarbon refrigerants (HFCs) such as R-134a and related molecules because their polar nature provides improved miscibility with the refrigerant in comparison to other lubricants such as mineral oils, poly-alpha-olefins, or alkylated aromatics. One example of such a polyol ester lubricant is disclosed in U.S. Pat. No. 6,221,272.
The physical characteristics for simple polyol esters are primarily derived from the structure of the acid component. Because there are a wide variety of commercially available carboxylic acids, simple polyol esters can be designed with specific physical characteristics that are optimized for a particular refrigeration system application. But for simple polyol esters there are limits to the simultaneous optimization of all desired properties. For instance, the optimum lubricant would be one that has high miscibility with the refrigerant at low temperatures to ensure good transport of the lubricant in the evaporator and other low temperature components of the refrigeration cycle, but very low or poor solubility of the refrigerant in the lubricant at high temperature and pressure in the compressor to minimize viscosity reduction of the lubricant by refrigerant.
Viscosity reduction of the lubricant by refrigerant at high temperatures and pressures dramatically reduces the hydrodynamic lubricating ability of the lubricant. Also, the lubricity and load carrying ability of a polyol ester lubricant is improved by using longer chain linear acids rather than shorter chain and/or branched alkyl groups. But the exact opposite is true for miscibility with HFC or fluorocarbon refrigerants (i.e., branched and/or shorter chained acyl groups improve miscibility). So there is a careful balance required to optimize both the miscibility characteristics of the lubricant with refrigerant at low temperature and solubility of the refrigerant in the lubricant at high temperature and pressure, while also maintaining the best balance of lubricity and load carrying ability of the lubricant. In addition, the negative impact on lubricity and load carrying ability of the lubricant will become more pronounced as refrigeration system manufacturers move to lower viscosity lubricants to improve energy efficiency.
One mechanism for improving the lubricity and load carrying ability of a refrigeration lubricant is to include anti-wear/extreme pressure additives. However, such additives may be undesirable since they can either precipitate out from the lubricant at low temperatures (as are encountered in the evaporator) or decompose to insoluble by-products at very high temperatures (as are experienced in the compressor). Such “drop out” of the additives from the lubricant can often lead to deposits on, or complete blockage of, the refrigerant system expansion device (thermal expansion valve, capillary, or needle valve) leading to a decrease in refrigeration performance or complete failure of the system. Additionally, for compressors with internal motors there is a potential for unfavorable reaction of the additives with the wire coatings used on the motors, leading to solubilization of the wire coatings in the system and eventual deposit on the expansion devise components.
Thus there is a need for a refrigeration lubricant that possesses high miscibility with the refrigerant over a wide temperature range of operation while also maintaining adequate lubricity and load carrying ability without the use of additives; provides protection against wear of refrigeration components and in addition improves the energy efficiency of the refrigeration system.
One potential way of addressing this need is to employ complex polyol esters, that is esters formed by the reaction of alcohols containing at least two —OH groups with polybasic carboxylic acids, normally in admixture with one or more monobasic carboxylic acids. Thus by virtue of their additional acid sites, polybasic acids offer the potential for tailoring the properties of the resultant esters to meet the varying requirements of an optimal lubricant.
For example, U.S. Pat. No. 5,096,606 discloses a refrigeration oil composition comprising (1) fluoroethane selected from the group consisting of 1,1,1,2-fluoroethane, pentafluoroethane, 1,1,1 trifluoroethane, and 1,1-difluoroethane and (2) an ester compound which is a reaction product obtained from (a) an aliphatic polyhydric alcohol having 1 to 6 primary hydroxyl groups, (b) a saturated aliphatic straight or branched monocarboxylic acid having 2 to 9 carbon atoms, or a derivative thereof and (c) a saturated aliphatic straight or branched dicarboxylic acid having 2 to 10 carbon atoms, or a derivative thereof, said ester compound having a kinematic viscosity at 100° C. of 1 to 100 cst.
In addition, U.S. Pat. No. 5,750,750 discloses a complex alcohol ester which comprises the reaction product of a mixture of the following:
a polyhydroxyl compound represented by the general formula: R(OH)n wherein R is any aliphatic or cyclo-aliphatic hydrocarbyl group and n is at least 2, provided that said hydrocarbyl group contains from about 2 to 20 carbon atoms;
a polybasic acid or an anhydride of a polybasic acid, provided that the ratio of equivalents of said polybasic acid to equivalents of alcohol from said polyhydroxyl compound is in the range between about 1.6:1 to 2:1; and
a monohydric alcohol, provided that the ratio of equivalents of said monohydric alcohol to equivalents of said polybasic acid is in the range between about 0.84:1 to 1.2:1;
wherein the complex alcohol ester exhibits a pour point of less than or equal to −20° C., a viscosity in the range between about 100-700 cSt at 40° C. and having a polybasic acid ester concentration of less than or equal to 70 wt. %, based on the complex alcohol ester.
Further, U.S. Pat. No. 5,853,609 discloses a refrigerant working fluid which remains in a single phase between about −40° C. and about 71° C., said working fluid comprising a substantially chlorine-free fluoro-group-containing heat transfer fluid that comprises at least one of pentafluoroethane, 1,1-difluoroethane, 1,1,1-trifluoroethane and tetrafluoroethane and a composition of matter suitable for serving as a lubricant base stock, said composition being a liquid with a viscosity between about 22.5 and about 44 centistokes at 40° C. and consisting essentially of a mixture of polyol ester molecules in which at least 85% of the monobasic acid molecules in the acid mixture consist of molecules having five or nine carbon atoms each, at least about 92% of the alcohol moieties are selected from the group consisting of alcohol moieties derived from pentaerythritol (PE) and dipentaerythritol (DPE) and at least about 92% of the acyl groups are selected from the group consisting of the acyl groups of all the straight and branched chain monobasic and dibasic carboxylic acids with from four to twelve carbon atoms each, said alcohol moieties and acyl groups being further selected subject to the constraints that (a) a total of at least about 7% of the acyl groups in the mixture are acyl groups of i-C5 acid; (b) the ratio of the percentage of acyl groups in the mixture that contain 8 or more carbon atoms and are unbranched to the percentage of acyl groups in the mixture that are both branched and contain not more than six carbon atoms is not greater than about 1.56; (c) the percentage of acyl groups in the mixture that contain at least nine carbon atoms, whether branched or not, is not greater than about 81; (d) not more than about 2% of the acyl groups in the ester mixture are part of acid molecules with more than two carboxyl groups each; (e) at least 60% of the monobasic acid molecules in the acid mixture consist of molecules having no more than ten carbon atoms each; and (f) a total of at least about 20% of the acid molecules in the mixture are one of the trimethylhexanoic acids; at least about 85% of the alcohol moieties in the esters are those of PE; and not more than about 7.5% of the acyl groups in the ester mixture are dibasic.
U.S. Published Patent Application No. 2005/0049153 discloses a lubricant composition comprising a complex polyol ester having: (a) a polyfunctional alcohol residue; and (b) a saturated or unsaturated dicarboxylic acid residue having from about 9 to about 22 carbon atoms. All the complex polyol esters exemplified have a viscosity in excess of 100 cSt at 40° C.
Thus, as will be seen from the above, most existing complex ester lubricants have a viscosity at 40° C. in excess of 100 cSt, whereas for many applications it is desirable to employ lower viscosity lubricants since these reduce the energy required to operate the refrigeration system. In addition, although there has been some limited work on low viscosity complex ester lubricants, these have in general required the use of very low levels of polybasic acid. Thus, with most polyol esters use of even low levels of polybasic acids increases the molecular weight significantly and leads to a rapid increase in viscosity.
According to the present invention, a complex polyol ester has now been developed that exhibits an advantageous combination of low kinematic viscosity and high viscosity index. The low viscosity provides the ester with good energy efficiency during start up, whereas the high V.I. ensures that the ester has acceptable viscosity at operating temperature to provide good lubricity and wear protection of refrigeration system components without the need for excessive amounts of antiwear/extreme pressure additives. In addition, the present low viscosity, high viscosity index complex polyol esters can be blended with higher viscosity traditional polyol esters to obtain intermediate viscosity grade lubricants with improved properties over simple polyol esters.